The present invention relates generally to a method and apparatus for screen printing. More particularly, it pertains to printing a screen paste through a medium onto a substrate.
Circuit patterns are produced by a number of manners, including screen printing. Typically, in a screen printer a screen having a printing pattern, is disposed over an object to be printed. Printing ink is coated onto the screen, and then spread by the use of a wiper also disposed over a top surface of the screen. The wiper assists in the printing of the pattern onto the object to be printed. One example of a conventional screen printer is illustrated in U.S. Pat. No. 5,176,076 issued to Azuma on Jan. 5, 1993.
Typical screen printers, however, can result in excessive material costs. The printing paste is disposed on the screen for each application and can dry out and solidify rapidly due to the repeated working of the paste, as well as its exposure to air. In addition, cleaning issues also occur as the paste dries on the screen in between uses. Other areas which challenge conventional screen printers is uniformity in the thickness of application of the printing medium. The printing paste undergoes changes such as solidification, oxidation, etc. from being left on the screen when not in operation, which results in an undesirable printing effect. Thus, the paste must be removed from the screen and replaced with new printing paste. Replacement of the paste requires additional material and additional labor. Furthermore, solidification of the paste can result in variations in thicknesses of the paste application leading to excessive use of the paste, thereby further adding to the overall cost of the unit.
Accordingly, what is needed is a screen printer which overcomes the above waste and cleaning problems. What is further needed is a screen printer which assists in preventing the drying of printing paste between applications. What is also needed is a screen printer which provides a uniform application of material.
A screen printing apparatus is disclosed which includes a substrate which is adapted to receive a printing substance. The substrate can include devices such as a wafer or a printed circuit board. The apparatus further includes a printing screen having a top surface and a bottom surface, where the top surface is disposed toward the substrate. A retention device is also coupled with the substrate, and the retention device retains the substrate over the top surface of the printing screen. Further included is a reservoir for containing a printing paste, such as solder, where the reservoir has a plurality of side surfaces and a bottom surface. The reservoir is disposed below the bottom surface of the printing screen.
In one embodiment, a wiper assembly is associated with the reservoir, where the wiper assembly includes a wiper and a reciprocating assembly. In another embodiment, the printing screen is flexed toward the substrate, and/or the top surface of the printing screen contacts the object to be printed. A retention device, in yet another embodiment, retains the substrate over the printing screen mechanically or by a vacuum. Alternatively, in another embodiment, a movement assembly is provided which is coupled with the printing screen. The movement assembly is adapted to provide paste on the printing screen, for example, in the form of convex bumps, or alternatively, to force paste through the printing screen. In another embodiment, a heater is thermally coupled with the reservoir.
A screen printing apparatus includes a substrate which is adapted to receive a printing substance. The substrate can include devices such as a wafer or a printed circuit board. The apparatus further includes a printing screen having a top surface and a bottom surface, where the top surface is disposed toward the substrate. A retention device is also coupled with the substrate, and the retention device retains the substrate over the top surface of the printing screen. Further included is a reservoir for containing a printing paste, such as solder, where the reservoir has a plurality of side surfaces and a bottom surface. The reservoir is disposed below the bottom surface of the printing screen. Associated with the reservoir is a wiper assembly, where the wiper assembly includes a wiper and a reciprocating assembly. In another embodiment, the printing screen is flexed toward the substrate, and/or the top surface of the printing screen contacts the object to be printed. A retention device, in yet another embodiment, retains the substrate over the printing screen mechanically or by a vacuum.
A further embodiment includes a method of screen printing onto a surface of a substrate. The method comprises disposing a bottom surface printing screen within a reservoir of a printing medium, contacting a top surface of the printing screen with a substrate; and translating a wiping mechanism over the printing screen, causing the printing medium to be disposed on the substrate. In another embodiment, the method further includes flexing the printing screen toward the substrate with the wiping mechanism.
The printing apparatus advantageously aids in producing consistent bump heights, particularly when trying to obtain a bump higher than the width of the bump. The viscosity of the printing paste is also better controlled, in part, due to the use of the heater. In addition, by containing the print medium in a reservoir below the screen, automation of the screen printing operation is much easier since it is not necessary to dispense print medium onto the screen between applications.
The printing apparatus provides a combination of pressure squeegee forces that is not currently available in standard screen-print equipment. This adds another control of the print medium, which would be useful to dispense accurate and consistent solder paste height and width characteristics. By utilizing the additional pressure control, it is possible to successfully use print medium with higher variations in viscosity and thixotropic index.
The screen printing apparatus also dispenses a predetermined amount of printing paste having a predetermined temperature onto the printing screen in each printing cycle. The apparatus assists in preventing the printing paste from being solidified and maintains a desired degree of viscosity. In addition, the apparatus aids in preventing the printing paste from being excessively applied onto the surface of the screen, which would otherwise result in uneven printing. Furthermore, the below-described printing apparatus minimizes the amount of printing paste which is wasted.
These and other embodiments, aspects, advantages, and features of the present invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art by reference to the following description of the invention and referenced drawings or by practice of the invention. The aspects, advantages, and features of the invention are realized and attained by means of the instrumentalities, procedures, and combinations particularly pointed out in the appended claims and their equivalents.